1. Field of the Invention
This invention relates to a thermocouple adapted for accurate and reproducible measurement of high temperatures in inert, oxidizing or reducing environments, gases, or vacuum, and more particularly to a thermocouple leg comprising a noble metal-Al alloy.
2. Description of the Prior Art
High melting, noble metal thermocouples made of e.g., platinum (Pt), rhodium (Rh), palladium (Pd), iridium (Ir) , etc., and alloys thereof are known in the art. For example, the most widely used thermocouple for measurement of temperatures above 1000.degree. C. is Pt-Pt13Rh, where one leg of the thermocouple is made of a wire or thin film of Pt and a second leg of the thermocouple is made of a wire or thin film of Pt13Rh (i.e., an alloy of platinum and rhodium containing 13 wt. % rhodium). The emf-temperature response of a Pt-Pt13Rh thermocouple, the basis of temperature measurement via thermocouples, is high (e.g., about 12.5 mv at 1200.degree. C.) , and its oxidation resistance is good. The Pt-Pt13Rh thermocouple can be used with minimal drift (i.e., a change in emf with time due to any cause such as composition change, oxidation or chemical attack) up to 1500.degree. C. Other precious metal elements, e.g., Pd and Ir, or alloys thereof with Pt are also useful thermocouples. Such thermocouples are not widely used because they are more susceptible to oxidation than Pt, and degrade by drift caused by selective oxidation.
On the other hand, Pt-modified pack aluminide coatings and the oxidation properties of these coatings in Pt-Al bulk alloys are known in the art. However, none of this technology concerns coating of Pt wires, making Pt-Al alloy wires, or using Pt-Al wires as thermocouple devices. The essence of the Pt-modified aluminide coating is that a thin layer (less than about 0.025 mm) of Pt is plated or CVD coated onto a nickel-base superalloy and then aluminized to form a Pt-Al rich outer layer in a NiAl coating. The Pt-Al phases are more diffusionally stable than NiAl, and serve as a long-term aluminum reservoir for protective Al.sub.2 O.sub.3 scale formation. Pt-Al oxidation studies have shown that about 5-10 a/o Al (atom %) is needed to form a protective Al.sub.2 O.sub.3 scale. Excellent oxidation resistance is obtained for these alloys.
A disadvantage of Pt-Pt13Rh as a thermocouple is its use of the strategic precious metal Rh. Rh is expensive and its availability is unreliable. Rh is also somewhat more prone to oxidation than Pt, so that alloys thereof suffer selective elimination of Rh resulting in a change in the emf of a Pt-Pt13Rh thermocouple over time (i.e., a temperature measurement inaccuracy). In reducing atmospheres, contact with any silica-containing material, e.g., silica-based refractories and SiC or Si.sub.3 N.sub.4 ceramics, can result in low melting Pt-Si compounds and destruction of the thermocouple.
The following patents relate to high temperature thermocouples, including Pt-Rh elements or analogous alloys which utilize rare and expensive metals or other metals.
U.S. Pat. No. 3,901,734 to Sibley et al relates to a thermocouple in which the positive element consists of an iron-nickel alloy and the negative element consists of a copper-nickel alloy. The compositions of these elements are such that inexpensive lead wires of copper, for example, may be used without any significant temperature error, where the thermocouple is used to monitor a temperature of about 1600.degree. F. and the lead wire-thermocouple junction is at a temperature not exceeding about 400.degree.-500.degree. F. Particularly, the emf output of the Sibley et al thermocouple between 32.degree. F. and about 500.degree. F. is said to approach substantially zero when the compositions of the positive and negative thermoelements are properly selected.
U.S. Pat. No. 4,402,447 to Przybyszewski et al relates to a method for bonding a platinum base metal lead wire to a thin platinum alloy film (typically Pt or Pt10Rh) resting on a thin alumina insulating layer adhered to a metal substrate. Typically, the platinum alloy film forms an element of a thermocouple for measuring the surface temperature of a gas turbine airfoil.
U.S. Pat. No. 4,984,904 to Nakano et al relates to a platinum-platinum/rhodium alloy thermocouple housed in an immersion protection tube used to monitor the temperature of molten metal. The immersion protection tube formed of alumina-graphite is said to protect the thermocouple from high temperature exposure to a reducing atmosphere.
U.S. Pat. No. 5,043,023 to Bentley relates to a metal sheathed thermocouple cable comprising an oxidation-resistant nickel-based alloy. The thermocouple cable is said to have excellent thermoelectric and mechanical stability at high temperatures (beyond about 900.degree. C.) , and is said to avoid premature failure due to oxidation that occurs in "bare-wire" thermocouples.
The following technical publications relate to platinum modified aluminide diffusion coatings.
M. R. Jackson et al, "The Aluminization of Platinum and Platinum-Coated IN-738", Metallurgical Transactions A, Vol. 8A, pages 1697-1707 (November 1977) relates to the chemistry and morphology of aluminide coatings formed on platinum and platinum-coated IN-738. These coatings are used to protect Ni-base superalloy gas turbine components from oxidation and hot corrosion attack.
J. S. Smith et al, "Platinum Modified Aluminides-Present Status", presented at the Gas Turbine and Aeroengine Congress and Expedition-Brussels, Belgium (Jun. 11-14, 1990) provides an overview of the development of platinum modified aluminide diffusion coatings used to impart oxidation and hot corrosion resistance to nickel-base superalloys. This paper discusses various coating morphologies and application of a low pressure chemical vapor deposition (low pressure CVD) process for production of platinum modified aluminide gas phase coatings on gas turbine components.